Melbourne Examines the Fading Problem by Measuring Signal Strength

Radio Record, Volume I, Issue 28, 27 January 1928, Unnumbered Page

Melbourne Examines the Fading Problem by Measuring Signal Strength

Victoria has its fading problem just as has every broadcasting station, and some areas are particularly bad. The following article will give New Zealand listeners a glimpse of her problem, and of the facts in connection with transmission which are being discovered by the steady measurements of signals over the area surrounding 3L0O.

By

R. O.

CHERRY

, M.Sc, RESEARCH PILYSICIST, UNIVERSITY OF MELBOURNE,

ime Sill ned SALUT SAUNT IRE PAA sed Sal HHI ws HU | CRTHER measurements of the intensity of the broadcast signals of 3L0, Melbourne, have been made in order to discover what service listeners-in are receiving, and in order to elucidate the transmission of wireless signals. Incidentally, information is obtained by such measurements of the efficiency of the broadcasting station at Braybrook. ‘The writer published recently a pamphlet containing an account of the measurements which he had made of signal intensity of 3L0 up to a distance of eight miles from the broadcasting station. More sensitive measuring apparatus has been devised and the measurements have been extended up ‘to 30 miles from Braybrook, and some readings have been taken up to a distance of 60 imiles from the sending aerial. Several mathematical theories of the transmission of wireless waves over the surface of the earth have been developed, and it is from the results of measurements at distances up to 50 or 70 miles that tests of the validity of these theories can be obtained. Such measurements will help to elucidate, too, the cause of fading of wireless signals, a natural phenomena which causes serious trouble to listeners-in situated in parts of Victoria remote from

Melbourne when receiving metroFolitan stations. The service which listeners-in are receiving from 8L0, and how it compares with standards Iaid Gown by the experts of the British Proadcasting Corporation and of the. Radio Corporation of America will be first discussed, and following that the question of the transmission of the wireless waves will be referred to. Before taking up these matters a word may be said about the method of measurement used. In the measurements up to miles from the station a measuring apparatus consisting of a loop condenser and yilye voltmeter was used, In these tests this simple apparatus has been made more sensitive without being made very much more elaborate. ‘The great advantage of the apparatus is its portability, and all the obseryations which haye required the Writer to travel some 1500 miles have been obtained using a motorcycle, The loop used is collapsible, and it and the rest of the

apparatus can be carried on the motorcycle. More elaborate signal intensity measuting apparatus are generally used, such as that of the Weston ‘Telephone Co., which has recently been purchased by the Research Laboratory of the Postmaster-General’s Department, but this apparatus requires a special motor-car for its trausport, ‘The simpler apparatus used by the writer is able to detect the signal from 40G, Brisbane, and observe, too, the fading of that signal. Although it is not intended at present to study the problems of transmission at these very great distances, it is interesting to find that a simple apparatus costing but a small fraction of the larger instrument will do the same work. It has the additional advantage that, beiug very simply constructed, there is practically nothing to go wrong. In order to judge the nature of the service given by a broadcasting station to listeners-in, certain arbitrary standards have been laid down which have been taken as fairly representative of conditions prevailing in both Great Britain and America. In the previous paper by the writer, standards laid down by the Radio Corporation of America were quoted, and it is interesting to find that essentially the same figuies are found to be necessary for Einglish conditions. Since the question of these standards is all in:portant from the listener-in’s point of view, we will guote Captain Keckersley’s figures, as they appeared in the "Wireless World and Radio Review" :-

1. Field strengths of 30 to 100 millivolts/metre. Practically a "wipe out"? area as far as other stations are concerned, 2. Vield strengths of 10 to 30 millivolts/metre. An ‘A’ seryice area. The iistenet-in practically assured of an uninterrupted service, however near-within limits-to a source of electrical disturbance. 3. Field strengths of 5 to 10 millivolt/metre, A ‘‘B" service area. Assuming a good aerial, crystal reception is satisfactory, ‘There may be slight interference. 4. Field strengths of 2.5 to 5 millivolts/metre, A "C" — service area. Interference begins to present a serious problem. 5. Field strengths below 2.5 millivoltsjmetre. Reception becomes unsatisfactory. As these figures will not mean much to many readers, we will translate them into corresponding distances from the station 38L0. The "wipe out’? area will extend about 10 miles from Braybrook and will include all the western and northern suburbs of Melbourne and most of the inner suburbs to the east, as far as Histernwick, Armadale, Kew, and Ivanhoe. An "A’’ service area, i.e., an uninterrupted service, will extend from 10 miles to 20 or 25 miles, according to direction. ‘The following towns are about the limit of this area:-Little

River, Bacchus Marsh, Wallan, Ringwood, Dandenong, and Mornington, The "B" service area in which there is satisfactory service on a crystal set with a good aerial is further out than has yet been fully investigated, but from the readings already taken we may say that an area within 80 or 40 miles’ radius will be given this type of service. Since about 70 per ceut. of the population of the State live within this area, it appears that the broadcasting station is giving a service where crystal reception can be generally satisfactory to the major portion of the population, The boundaries of the *'C" seryice * area will probably be indefinite since hills and mountains are encountered to the west, north, and east of Melbourne within 80 miles. ‘These will have the effect of disturbing the normal distribution of field strength, introducing local peculiarities, As mentioned above, these standards are based on Continental and American experience, and it is assumed that such standards will also apply to Australia. Comparison of these distances with those for stations in other parts of the world, brings out the fact that the area Siving a satisfactory service is very large for a station rated at 5 kilowatts. This result was to be expected from the first series of readings. One new and important fact that has come out of the work so far completed is the peculiar effect that hills have on the field strength distribution. From a number of observations, it appears that at the top of the hill the field strength AMI a AZ AWA TUE ie Al Ut te MO

iS much greater than it is on the level ground at the hase of the hill. In one case, the ratio was nearly 2,1 on a hill 300 feet high, and effects of the same order of magnitude have been observed elsewhere. The theory of this effect is far from complete and more observations Will he necessary before the full details haye been examined. ‘This "hill effect," as we may call it, Plays an important part in deter. mining the nature of the service given to the Jisteners-in since the Flound rises steadily to the west, north, and east of Melbourne, causing a natural increase in field strength over that normally expected. l'rom the observations already taken, it appears that it is not merely the heieht of the hitl that determines the increase in field strength. hut also its outlook in the direction from which the wireless signals are coming, ‘Thus at Studley Park, at the ton of a steen slone facing in the direction of Bravhrook, the field strength is proportionately 25 per cent. stronger than it is at the ton of Oneen’s College tower, 100 feet hich, standiee on eronnd higher itself than Studley Park. Further wel on there prints is proceecin. All AMT a CAM t J UAL eneants comme a wantse ET i at M .)